1. Field of the Invention
The present invention relates generally to an unmanned vehicle control system and the method, and more specifically to a steering device control system and the method incorporated with the unmanned vehicle control system. The unmanned vehicle is a forklift truck, for instance, which is automatically remote-controlled so as to travel along a fixed guide line in response to various command signals transmitted from a fixed host computer. The unmanned vehicle is used for conveying material, semi-finished products, etc. in an unmanned factory, warehouse, etc.
2. Description of the Prior Art
In general, an unmanned vehicle is driven along fixed guide lines. An alternating current having an appropriate frequency (e.g. 10 kc) is passed through the guide lines to generate a magnetic field near the guide lines. A pair of magnetic field detecting coils are disposed symmetrically with respect to the longitudinal axis of the vehicle body in order to detect a vehicle offset produced when the vehicle skews or is misaligned away from the guide line. To detect vehicle offsets (misalignment) from the guide line, the difference in magnetic field intensity between the two symmetrically-arranged coils is detected. In response to the detected difference signal indicative of vehicle offset, a steering device mounted on the vehicle is actuated by a steering motor so that a detected offset value may be reduced into within a dead zone (offset is no longer adjusted). The speed of the steering DC motor can be adjusted by a chopper circuit. The chopper circuit can generate a chopped current the duty ratio of which is freely adjustable by controlling the inputs of the chopper circuit. In other words, when the vehicle is misaligned to the right side, for instance, the duty ratio of the chopper circuit is calculated according to the magnitude of the detected offset, and the steering motor is driven in the direction that the offset is reduced at an appropriate motor speed.
In the prior-art steering device control system, however, since the dead zone is fixedly determined and additionally the speed of steering motor is simply adjusted according to the magnitude of the detected offset value irrespective of vehicle speed, there exist some shortcomings. The basic problems are: When the vehicle is travelling at high speed, the dead zone is reduced in reality and the vehicle misalignment is excessively adjusted repeatedly, so that the vehicle travels zigzag at high speed and additionally large rush currents will often be generated in the steering chopper circuit. The generated rush currents may damage the chopper circuit. On the other hand, in the case where the dead zone is determined wider, it is impossible to finely control the steering operation at low vehicle speed. In summary, in the prior-art steering control system, it is impossible to stably correct the steering misalignment throughout a wide vehicle speed range.